Recently, there have been growing demands for recycling waste paper with a view to saving global resources and preserving the environment, and increasing the range of application of recycling has become an important issue. While conventional recycled pulp has generally been used for newsprint/magazine paper etc., a demand has grown recently for the production of recycled pulp having high brightness and low residual ink; which is to be achieved by advanced treatment of waste paper with a view to expanding a range of application of recycled pulp.
Methods for recycling waste paper typically comprise a step of stripping ink from pulp fibers and a step of removing the stripped ink. More specifically, the mainstream method comprises promoting stripping of ink from pulp fibers and fragmentation thereof by hydrodynamic shear forces or the friction force between pulp fibers in a pulper and subsequently removing ink by flotation and/or washing. During this process, waste paper is typically treated at basic pH by adding deinking chemicals such as sodium hydroxide, sodium silicate, oxidizing bleaches and/or reducing bleaches and deinking agents, if needed. Advanced treatment methods for waste paper include, for example, promoting stripping/fragmentation of ink by mechanical force after the ink stripping step or ink removal step or removing ink again and then (patent document 1) stripping ink that could not be removed by the previous treatment and remains on pulp fibers.
However, diversification of waste paper has led to increased inclusion of offset inks deteriorated over time to an advanced stage of oxidative polymerization, toner inks thermally fused to pulp fibers, UV-curable resin inks cured by treatment during printing, etc., which cause the problem of insufficient stripping of ink during recycling that results in a significant impairment in the final pulp quality due to the presence of unstrapped ink in the paper pulp. This problem has been addressed by treatment under higher mechanical load or at higher temperatures or by adding larger amounts of chemicals such as alkalis or deinking agents, but the increased mechanical load caused problems such as fragmentation of pulp fibers or increase of small fibers or twisting of pulp fibers resulting in a loss of paper strength or size stability or paper curling; and the increase of steam costs for high temperatures or chemical costs caused problems of increased costs. Another approach is to repeat multiple cycles of step of stripping ink by mechanical force and the step of removing ink by flotation and/or washing until a desired pulp quality is obtained, but this approach requires a very large capital investment and invites an increase in drainage load so that it is considerably disadvantageous from the aspects of cost, energy and environmental conservation and has not been widely used. Thus, especially toner prints and UV-curable resin ink prints have been used for only paperboards and household paper, but have not been positively used as raw waste paper materials for printing paper, specialty paper for communication and newsprint paper.
Moreover, improvements in the utilization rate of waste paper has enabled pulp fibers per se to be recycled a greater number of times, and with repeated recycling pulp fibers are more severely damaged as reported by Okayama et al. (Takayuki Okayama, The 7th Basic Lecture about Pulp, Waste Paper Pulp (Part 2), Edited by Japan Technical Association of the Pulp and Paper Industry, p 101-111, 2002). It is known that the hydrogen bonding capability of pulp fibers considerably decreases by heat drying, and when waste paper once dried is to be recycled, the hydrogen bonding capability must be improved by beating the waste paper with a refiner or the like to raise pulp fibers in order to compensate for the decrease in hydrogen bonding capability. However, the inner structures of pulp fibers are seriously damaged and turn into structures having lamellar or annular cracks during this process. Pulp fibers in such a state seem to be readily broken by friction between fibers due to mechanical force or contact with agitating blades during the process of recycling waste paper, thereby promoting fragmentation of pulp fibers. Thus, conventional techniques can not avoid damage to pulp fibers or an increase in costs such as energy consumption or drainage costs in order to produce high-quality pulp from diversifying waste papers.
Pulp fibers have conventionally been modified by beating pulp using a mechanical force to fibrillate pulp fibers so that the bonding area between fibers is enlarged by raising microfibrils on fiber surfaces to improve bonding strength. However, such a method deteriorates paper strength because of a decrease in fiber length resulting from cutting of pulp fibers per se.
Ink stripping performance has conventionally been improved by causing friction between pulp fibers under mechanical load such as kneaders and surface chemical action by surfactants called deinking agents. However, hard-to-strip toners and UV-curable resin inks are being used increasingly in recent years, which causes damage to pulp fibers per se due to the increased mechanical load.
For bleaching kraft pulp, chlorine bleaches have been used to decompose and remove organic coloring components derived from lignin or the like, but they are being replaced by bleaching without using chlorine or bleaching without using even chlorine bleaches because of the environmental impact of bleach wastewater, which causes problems of increased costs due to the decreased bleaching efficiency.
Thus, conventional techniques could not avoid damage to pulp fibers or cost increases such as chemical costs because of the mechanical load imposed on the entire pulp fibers in order to improve pulp quality.
The following Ukrainian non-patent documents use cavitation for producing waste paperboard pulp, but do not include deinking treatment.
Patent document 2 discloses a method for treating pulp via cavitation by passing a pulp suspension through a reactor having a special shape, but it was difficult to fully strip/remove ink by this method.